High sensitivity focal sensor for electron beam and high resolution optical lithographic printers

ABSTRACT

A three slit interferometric focus control device capable of focal positioning accuracies on the order of 300 angstroms. The device includes a zero path Michelson interferometer with reference and movable legs comprising, respectively, an annular reference mirror and the surface to be brought into position of best focus. A three slit plate positioned between the beamsplitter and energy detection device allows interference of light reflected off the movable and reference surfaces, creating an amplitude modulation signal. The energy detection device will measure a minimum energy intensity at the position of best focus.

BACKGROUND OF THE INVENTION

The present invention relates to high resolution optical lithographicsystems, and, more particularly, those used in the fabrication ofintegrated circuits.

The fabrication of very high density integrated circuits ("IC's") isdependent upon the availability of high resolution lithographic systemscapable of resolving and exposing the narrow circuit linewidths onto thephotoresist masks used during the manufacturing process. At present,there are several different designs for optical lithographic systemscapable of producing a minimum linewidth resolution of about one (1)micron, i.e., one micrometer. Currently, the IC industry is seeking aproduction worthy means of exposing 0.5 micron linewidths to reduceelectronic component size as required by high technology applications.

One optical lithographic design utilizes a reflective optical (1:1projection ratio) projection system to image a narrow arc of the desiredmask image onto the circuit substrate wafer and photoresist. As thesystem simultaneously scans the mask image and object (wafer) planes, atwo dimensional image of the mask is exposed on the photoresist coatedon the circuit wafer. This system, which is designed to maintain aconstant optical pathlength between mask image and wafer surface, issubject to inaccuracies caused by small variations in wafer thickness,as well as variations in flatness of the wafer, wafer holder, mask andreflective optics. These mechanical problems, coupled with thedifficulty of producing accurate 1:1 masks and diffraction problemsassociated with imaging these narrow lines, have limited the resolutionof these 1:1 lithographic printers to approximately 2.0 microns.Production linewidths of 1.25 microns are achieved in these systems byusing short wavelength ultraviolet sources (330 nanometers (nm)).Ultimately, the performance of this system design is limited byaccumulated mechanical positioning errors and diffraction problems.

Another optical lithographic system design utilizes a reduction ratio inthe projection optics (commonly 10:1), and a precise mechanical stagecontrolled by a laser interferometer to replicate an enlarged mask onthe photoresist and wafer surface. This system relieves mask fabricationlimitations of the 1:1 system, minimizes mechanical positioning errors,and reduces the errors caused by diffraction problems. Since the fieldof view of this optical system is smaller than the corresponding 1:1system, linewidths as fine as 0.75 micron are possible and have beendemonstrated in the laboratory. Production linewidths between 1.0 and1.25 microns are claimed.

As an alternative, electron-beam lithographic systems have beendeveloped which are capable of resolving 0.1 micron linewidths. However,these systems have several disadvantages which would make themundesirable if an optical system capable of 0.5 micron resolution wasavailable. First, the use of the electron-beam systems require that thewafer or photoresist be placed in a vacuum during exposure. As a result,the cycle time for creating each IC increases, since, for each circuitlayer, the wafer must be placed in the system, the system evacuated, thevacuum brought down and the wafer exposed and then replaced in thesystem. Contamination becomes a problem because of the increasedhandling of the wafer during IC fabrication. In addition, the costs ofelectron-beam systems are excessive in comparison to high resolutionoptical lithographic systems.

It is accordingly a primary object of the present invention to providean improved focal sensor for high resolution lithographic systems.

SUMMARY OF THE INVENTION

The above and other objects of the present invention are achieved byproviding an illumination system utilizing a deep ultraviolet ("UV")source in conjunction with a high reduction ratio (e.g., 10:1)reflective optical system, a fine focus sensor, and a UV sensitivephotoresist material. In an alternate embodiment, a refractive opticalsystem with a high reduction ratio replaces the reflective projectionsystem.

Since the maximum theoretical resolution of an optical system isdirectly related to the wavelength of the illumination source, thesimplest way to increase resolution is to lower the wavelength of thesource. The use of a reflective system removes all source wavelengthrestrictions and eliminates chromatic aberrations such that the lowestwavelength source can be used where matching photoresists are available.Current 1:1 reflective systems utilize near UV sources, but have beenlimited by mechanical positioning errors.

In the present invention, a very deep UV source (200 to 250 nm), forexample, either an Excimer NF₃, Argon or Krypton laser or a suitablelamp source, such as an arc lamp, is used to expose the mask pattern ona UV-sensitive photoresist/IC surface. A dynamic coherent opticalcondenser system is used to increase the system resolution and contrast(as measured by the Modulus Transfer Function, or "MTF"). A dynamiccoherent optical condenser system is described by Cronin, Pinard andSmith in U.S. Pat. No. 3,770,340 and by D. J. Cronin and A. E. Smith inan article entitled "Dynamic Coherent Optical System", OpticalEngineering magazine, Volume 12, at page 50, March/April, 1973. Thisincreased contrast is significant, first, because most photoresistsrequire sixty percent (60%) image modulation to record the exposure, andsecond, because it allows a reduction in the numerical aperture of thelens, thereby decreasing cost and lens design complexity.

Similar improvements in resolution and contrast may be obtained by usingan annular reflective element with central obscuration in combinationwith an annular dynamic coherent imaging system. An annular dynamiccoherent source may be obtained by utilizing a spinning prism or mirror,a coupled pair of acousto-optic modulators tuned to cause circularmotion of the focused laser spot, or multiplexed holograms incombination with the desired laser source. A fourth approach, utilizingan annular fiber optic bundle to create a condenser equivalent to thedynamic coherent annular source may be used. This condenser has anadvantage in that it is relatively inexpensive, and that it has nomoving parts, and thus creates no mechanical vibrations which mightaffect the focus or alignment subsystems.

To maintain the contrast and resolution requirements, the depth of focusand focus accuracy must be matched to the operating wavelength andnumerical aperture of the system. Current commercial opticallithographic systems utilize various types of focal sensors (pneumatic,intensity profiling, focus wedge or "fly's" eye lens) to achieve focalaccuracies of ±0.25 microns. To achieve 0.5 microns resolution on thephotoresist, focal accuracies between 300 and 2000 angstroms arerequired. The present invention may use a three-slit interferometricfocal sensor to overcome prior focus accuracy limitations. This type ofdevice is more accurate since it works on the principle of amplitudemodulation rather than detection of interferometric fringe shifts.

To insure proper registration of successive exposures of the mask andIC, the system must be aligned to within ten percent (10%) of thedesired linewidth, approximately ±0.05 microns. Although currentproduction systems are only capable of alignment accuracies on the orderof 0.25 microns, research systems using laser interferometric orintensity profiling and image processing techniques have demonstratedalignment precision of ±0.025 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects of the present invention are achieved in theillustrative embodiment as described with respect to the Figures inwhich:

FIG. 1A shows a block diagram of the optical imaging system withindirect focus control and

FIG. 1B shows a block diagram of a through-the-lens focus control devicefor use in the system of FIG. 1A.

FIG. 2A shows a schematic of a dynamic coherent optical condensingsystem,

FIG. 2B shows an equivalent annular fiber optic condensing system, and

FIG. 2C shows an example of the fiber optic assembly of such condensingsystem.

FIGS. 3A and 3B show schematic views of reflective and refractiveoptical projection systems, respectively.

FIG. 4A shows a schematic of a three-slit interferometric focal sensor,and

FIGS. 4B and 4C show the intensity distribution measured when the deviceis in and out of focus, respectively.

FIG. 5 shows the improvement in modulation transfer function whichresults from use of a dynamic coherent condenser in combination with aCassargrain-type reflective optical system design of the subjectinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring to FIG. 1A, the photolithographic system of the presentinvention includes a deep UV illumination source 10 in combination withdynamic coherent condenser optics 19, a circuit design mask 11, opticalprojection system 12, coarse focus control 13, fine focus control orsensor 14, an IC wafer 26 with UV sensitive photoresist 15, anelectromechanical stage 16 which holds wafer 26, an alignment system 17,and an electronic control unit 18. Dotted line 25 designates the imageplane.

In one application, the described components are selected such that theoperational photolithographic system is capable of exposing, forexample, approximately sixty (60) wafer levels per hour on a 4-inchdiameter IC wafer 26. The image field of projection lens 21 isapproximately ten (10) millimeters by ten (10) millimeters square sothat one hundred (100) separate exposures are required for each waferlevel. In such exemplary system, each exposure requires approximately0.25 seconds, including positioning, focus, alignment, and exposuretime.

The illumination source 10 is a deep UV source, either a laser or arclamp. Any deep UV source capable of producing enough power to triggerthe photoresist mechanism at the plane of photoresist 15 in less than a0.25 second period may be used. As an example, source 10 might comprisea cadmium arc lamp with appropriate filters, or an Excimer NF₃ (248 nm),Argon (231 nm), Krypton (219 nm) or Argon Floride (193 nm) laser.

A dynamic coherent optical condenser 19, or its equivalent shown inFIGS. 2A and 2B, is used to reduce edge ringing, speckle andinterference typical of coherent imaging systems, while providingincreased contrast and resolution, as well as a high cut-off frequencytypical of incoherent imaging systems. This significant increase incontrast and resolution allows approximately a thirty percent (30%)decrease in numerical aperture of the optical system, which reduces thedesign complexity and cost of the optical system. This reduction innumerical aperture increases the working depth of field of the system.

Typically, a dynamic coherent condenser comprises a coherent source atthe focal point of a condenser which will illuminate the optical system.The coherent beam is rotated about the edge of the pupil of thecondenser optics, either by physically rotating the source, or byrotating a prism about the optic axis which deviates the beam in thedesired manner. As shown in FIG. 2A, a preferred design of condenseroptics 19 comprises the coherent beam 20 from deep UV source 10, incombination with the condensing optical element 21 and preferably astatic device 22 for deviating beam 20 around the pupil edge of element21. The static device for deviating beam 20 is required to eliminatevibration which might disrupt system alignment and focus subsystems. Forexample, device 22 might comprise two (2) orthogonally orientedacoustic-optic crystals.

As described in the article by Cronin, DeVelis and Reynolds, entitled"Equivalence of Annular Source and Dynamic Coherent Phase ContrastViewing Systems", Optical Engineering magazine, Volume 15, No. 3, atpage 276, May/June, 1976, an incoherently illuminated annulus providesthe same performance improvements as the coherent condenser designsdescribed above. In an alternate embodiment, as shown in FIG. 2B,condenser optics 19 comprises a coherent beam 20 from deep UV source 10,a fiber optic bundle 23 of individual fibers 24 and a condenser element21.

The fiber optic bundle 23 reshapes beam 20 from a Guassian to an annulardistribution at location 44, which illuminates the pupil edge ofcondenser element 21. The bundle 23 is bonded to match the shape of beam20 received at location 46, and is coated to reduce reflections, asrequired. Lengths of individual fibers 24 are randomized to destroytemporal coherence within the bundle, thereby creating the same effectas a dynamic coherent condenser. An example of such fiber optic assemblyis shown in FIG. 2C. By way of example, the assembly is designed toreceive a rectangular output (such as, for example, from an Excimer NF₃laser), which is converted to an annular output at location 44. Itshould be noted that the fibers 24 may be more closely packed atlocation 46 than that placement shown in FIG. 2C so that lightthroughput may be maximized.

The shaped beam 20 from condenser element 21 illuminates circuit designmask 11. Mask 11 comprises a transparent positive image of the desiredpattern to be lithographically printed on IC wafer 26; scaled to matchthe reduction ratio of the projection optics 12. A typical mask includesa quartz substrate with an opaque representation of the desired circuitdesign pattern. The image of mask 11 is projected via optical system 12onto photoresist surface 15 of IC wafer 26.

In one embodiment, as shown in FIG. 3A, optical system 12 is areflective telescope comprising a primary focusing mirror 27 withcentral obscuration 28, and secondary element 29. As described above,the enhanced MTF provided by the dynamic coherent condenser 19 can beused to great advantage in a lithographic system where the desiredlinewidth limit (0.25 microns to 0.5 microns) occurs at one-third thecut-off frequency of the optical system (at approximately the sixtypercent (60%) contrast level). The relative enhancement of the MTF isgreatest when a dynamic coherent condenser is used in combination with aCassagrain-type reflective optical system, as shown in FIG. 3A.Reflective designs also have the advantage that design complexity isreduced because of the absence of optical aberrations typical ofrefractive systems.

Where the linewidth design is tuned to approximately one-third thecut-off frequency to achieve fifty to sixty percent (50-60%) contrast atimage plane 25, the optimum improvement in MTF is obtained in areflective optical system where the diameter of the central obscuration28 is less than one-third the diameter of primary focusing mirror 27.Expressed in an alternate form; optimum reflective optical systemperformance occurs when:

    (V/3)<[(V/2)-(e/2)], or e<(V/3);

where V is the normalized cut-off frequency of the optical system and eis the ratio of the obscuration diameter to the clear aperture diameterof mirror 27. FIG. 5 shows the improvement in MTF for a system in whichthe diameter of central obscuration 28 is one-third the diameter offocusing mirror 27. Line 54 shows the theoretical MTF of aCassagrain-type reflective optical system which does not incorporate adynamic coherent condenser system. Line 52 shows the MTF of the sameoptical system in combination with a dynamic coherent condenser system.Inflection points 56 on line 52 occur at a relative spatial frequency of0.5 ±e/2, in this example, at relative spatial frequencies of 0.35 and0.65.

In the subject invention, the numerical aperture (NA) of the system 12is approximately 0.2 to 0.4, with a 10:1 reduction ratio between mask 11and image plane 25. The system will project 0.25 microns to 0.5 micronscircuit linewidths with fifty to sixty percent (50%-60%) contrast at theimage plane 25 with image plane distortion of less than 250 angstroms.The diameter of element 27 and obscuration 28 are approximately 4inches, and 1.3 inches, respectively. The field of view of system 12 isapproximately 1 cm², such that complete exposure of one level of a 4inch IC wafer 26 requires one hundred (100) separate exposures, and canbe completed in less than one (1) minute.

In an alternate embodiment, optical system 12 comprises a refractiveoptical system as shown in FIG. 3B. This system might comprise adouble-Gauss lens design corrected for distortion, as well as otheroptical aberrations.

IC wafer 26 is a semiconductor substrate, for example, silicon, uponwhich successive layers of circuitry are built, typically, by theprocess of diffusion doping. It is noted that a basic process oflithographic construction of an IC is described in an article by L.Shepard and B. Carlson, entitled "Photo, E-Beam, and X-Ray Lithography",Scientific Honeyweller, Volume 1, No. 4, page 1, December, 1980. A layerof UV sensitive photoresist 15, such as Hunt WX-159 or Hitachi tri-levelUV sensitive resist is applied to the substrate 26 by conventionaltechniques. The spectral sensitivity of the resist 15 is matched to thewavelength and power output of UV source 10.

The image of mask 11 is projected onto photoresist 15 of wafer 26 viaoptical projection system 12. To obtain the desired linewidth resolution(0.25 microns to 0.5 microns), the photoresist 15 must be located withinthe depth of focus, P, about the image plane 25 of the optical system12, where

    P=±0.4L(2f/d).sup.2,

where L is the operating wavelength of the source of illumination, f isthe focal length of the optical imaging system and d is the diameter ofthe optical imaging system. By way of example, a focal accuracy ofapproximately 500 angstroms, from the ideal image plane 25, is required.Because commercially available automatic focusing devices are onlycapable of focal accuracies on the order of ±0.25 microns, the subjectinvention uses one of these well-known devices as a coarse focus control13, and may use a three-slit interferometric focus sensor as a secondfine focus control 14.

Coarse focus control 13 might utilize an indirect focus sensingmechanism such as a pneumatic device, or might function in conjunctionwith optical projection system 12 as in the case of a "through-the-lens"focus control mechanism, such as the intensity profiling device as shownin FIG. 1B. Through the lens focus control systems which require anillumination source may use the same source 10 which exposes thephotoresist, or may use a separate monochromatic/laser source, which canbe turned on when the UV source is off. A common source focus controlsystem is described by H. E. Mayer and E. W. Loebach in "A NewStep-by-Step Aligner For Very Large Scale Integration (VLSI)Production", SPIE, Volume 221, page 9 (1980). A separate source systemis described by S. Wittekoek in "Step and Repeat Wafer Imaging", SPIE,Volume 221, page 2 (1980).

FIG. 4A shows the three-slit interferometric focus sensor 14. Thisdevice works on the principle of amplitude modulation rather thandetection of fringe shifts, and is, therefore, more accurate thanconventional interferometric positioning devices. Wafer 26 and mirror 32with a central hole form the legs of a Michelson interferometer wherethe surface of mirror 32 and the plane of best focus 25 are an equaloptical distance from beamsplitter 30.

Collimated laser beam 20 enters the device and is split by beamsplitter30, as shown, so that it illuminates both wafer 26 and mirror 32. Lightreflected off these two surfaces returns via beamsplitter 30 throughlens 34 to photodetector 35. The mirror 32, baffles 31 and three-slitplate 33 are positioned so that the central slit passes light reflectedoff the surface of wafer 26, and the two outer slits pass lightreflected off the reference surface of mirror 32. Lens 34 focuses allcollected light onto photodetector 35. Electrical signal 36 fromdetector 35 is evaluated by electronic control system 18, whichgenerates a feedback signal 37 to control the position ofelectromechanical stage 16 along axis 38.

Best focus is obtained when stage 16 is moved such that photoresist 15lies on plane 25 as shown. In this position the intensity distributionon detector plane 39 is shown in FIG. 4B. The size of detector 35 isselected so that the intensity of the central peak 40 can be measured.The system is in focus when the peak height is minimized as shown in 4B.The intensity differences which result when wafer 26 is moved from anout-of-focus position (shown in FIG. 4C) to an in-focus position (shownin FIGS. 4A and 4B) are calculated by electronic control system 18 toderive the feedback signal 37. Electronic control system 18 may compriseany well-known microprocessor or minicomputer system in combination withcomputer programs to control the alignment system, wafer and maskpositioning, as well as the focus control device.

The alignment subsystem 17 comprises well-known laser interferometers,intensity profiling or image processing devices capable of maintainingsystem alignment to within 500 angstroms. Typical alignment andelectromechanical wafer positioning devices are described in the articleby Mayer and Loebach, cited above.

Having described the invention, what is claimed as new and novel and for which it is desired to secure Letters Patent is:
 1. A three slit interferometric focusing device for use in combination with an optical projection system having a desired plane of best focus, said focusing device comprising:A. a substantially coherent source of illumination; B. an optical beamsplitter positioned in the path of the beam of said source of illumination, said beamsplitter operative to produce a first beam in a first direction and a second beam in a second direction; C. a first movable surface coupled for movement to a position of best focus which is at a first distance from said beamsplitter and which is oriented to receive said first beam, said position corresponding to the plane of best focus of said optical projection system; D. means for moving said first surface to said position of best focus; E. a substantially reflective second surface having an opening which is substantially at the center of said second surface, said second surface located a distance from said beamsplitter substantially equal to said first distance and said second surface oriented to receive said second beam; F. an energy detection system capable of measurement at the wavelength of said source of illumination; G. an optically absorbing third surface having an opening which is substantially at the center of said third surface, said third surface placed between said optical beamsplitter and said first movable surface in the path of said first beam so as to substantially absorb a portion of said first beam and so as to enable the remaining portion of said first beam to be received by said third surface; and H. an optically opaque fourth surface with three substantially parallel slits, said fourth surface positioned between said optical beamsplitter and said energy detection system in order to pass a portion of said beam from said source to said detection system.
 2. A device as in claim 1 wherein said source of illumination, first surface, second surface and energy detection system are positioned with said beamsplitter in the configuration of a Michelsen interferometer.
 3. A device as in claim 1 wherein said said three slits include a central slit and two side slits, one on either side of said central slit, and wherein said central slit in said fourth surface is positioned to pass, to said detection system from said source, only light which has reflected off said first surface.
 4. A device as in claim 3 wherein said two slits on either side of said central slit are positioned to pass, from said source to said detection system, only that light which has been reflected off said second surface.
 5. A system as in claim 1 wherein said detection system measures an amplitude modulation caused by interference of light passed through said three slits and also measures a minimum interference intensity when said first surface is in said position of best focus.
 6. A system as in claim 1 wherein said means for moving includes a servo system which uses the output from said detection system to calculate the amount and direction of movement required to position said first surface at said position of best focus, said servo system coupled to produce an output signal which controls said means for positioning said first surface in order to maintain said position of best focus.
 7. A device as in claim 1 wherein said source comprises a laser.
 8. A device as in claim 2 wherein said source comprises a helium neon laser.
 9. A device as in claim 1 wherein said detection system comprises:A. an energy focusing lens; and B. one or more photodetectors coupled to receive light from said focusing lens in order to measure said amplitude modulation signal.
 10. A device as in claim 9 wherein said photodetector(s) comprise photodiode(s).
 11. A device as in claim 9 wherein said photodetector(s) comprise photocell(s). 